Using an if statement with arrays

Question

At the comment step 4 I am trying to add the current array element to sum, compare the current array element to max_test and if it is larger, save it in the variable max_test. and compare the current element to min_test, if it is smaller save it in min_test. HOWEVER i keep on getting the errors

Grades5.java:55: error: bad operand types for binary operator '>'
        if (grades[r] > grades[max_test]) 
                      ^
  first type:  int[]
  second type: int[]

Grades5.java:57: error: bad operand types for binary operator '<'
        if (grades[r] < grades[min_test])
                      ^
  first type:  int[]
  second type: int[]

Grades5.java:59: error: bad operand types for binary operator '+'
           sum += grades[r];
               ^
  first type:  int
  second type: int[]

3 errors

The code:

 import java.util.Scanner;

public class Grades5
{
  public static void main(String[] args)
  {
int[][] grades = {
                   { 87,  96, 100},
                   { 68,  75,  72},
                   { 99, 100,  95},
                   {100,  96,  70},
                   { 75,  60,  79},
                 };
int how_many_grades = grades.length * grades[0].length;

// -----------------
// Output the grades
// -----------------
System.out.print("           ");
for (int i = 0; i < grades[0].length; i++)
  System.out.print("Test " + (i + 1) + "  ");
System.out.println("Average");
for (int r = 0; r < grades.length; r++)
{
  int sum = 0;  // Sum of one student's tests

  // -------------------
  // Process one student
  // -------------------
  System.out.print("Student " + (r + 1) + "  ");
  for (int c = 0; c < grades[r].length; c++)
  {
    System.out.printf("%6d  ", grades[r]);                                                        // Step 1
      //sum += grades[c];                                                                     // Step 2
  }
  System.out.printf("%7.2f\n", (double)sum / grades[r].length);
}

// ----------------
// Output a summary
// ----------------
int max_test,  // Maximum test score
    min_test,  // Minimum test score
    sum = 0;   // Sum of all student tests

max_test = min_test = grades[0][0];    // Step 3
for (int r = 0; r < grades.length; r++)
{
  // -------------------
  // Process one student
  // -------------------
  for (int c = 0; c < grades[r].length; c++)
  {
                                     // Step 4
    if (grades[r] > grades[max_test]) 
      max_test = c; 
    if (grades[r] < grades[min_test])
       min_test = c;
       sum += grades[r];
  }
}
System.out.println("Highest test score: " + max_test);
System.out.println("Lowest  test score: " + min_test);
System.out.printf("Average test score: %.1f\n",
                  (double)sum / how_many_grades);
  }
}

Think about what grades[r] and grades[max_test] are, and what that means from your error message. It says you are trying to do int[] > int[], essentially compare two integer arrays, which is probably not what you intended to do. — nickb
– nickb, Commented Apr 5, 2016 at 20:59
It should be grades[r][c], not grades[r], because it is a bidimensional array. — Jose Luis
– Jose Luis, Commented Apr 5, 2016 at 20:59
You cannot simply change your question just because you got answers that fixed your original problem. Please accept an answer and ask another question if you think you're stuck - but be aware that the community does not really welcome problems which can be avoided by just using a debugger (at least if the problem is trivial). — Stefan Falk
– Stefan Falk, Commented Apr 5, 2016 at 21:38

Stefan Falk · Accepted Answer · 2016-04-05 21:05:42Z

1

Your problem is that grades[r] is not an integer. It is an array of integers. You'd have to address two indices

sum += grades[i][j];

in order to get rid of the compilation error.

It appears to be the same for the other errors. In general you can imagine it as follows:

grades[1] -> { 87,  96, 100}
grades[2] -> { 68,  75,  72}

and

grades[1][1] -> 87;
grades[1][2] -> 96;

etc ..

answered Apr 5, 2016 at 21:05

Stefan Falk

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Comments

OneCricketeer · Accepted Answer · 2016-04-05 21:14:37Z

1

If the grades for one test are horizontal in your array, then you only need two loops, not three.

for (int test = 0; test < grades.length; test++) {
    System.out.print("Test " + (test + 1) + "  ");

    System.out.println("Average");
    int sum = 0; // Sum all grades on this test
    for (int student = 0; student < grades[test].length; student++)
    {
        System.out.print("Student " + (student + 1) + "  ");
        sum += grades[student];  
    }
    System.out.println();
    System.out.printf("%7.2f\n", (double)sum / grades[test].length);
}

answered Apr 5, 2016 at 21:14

OneCricketeer

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Comments

Andrew · Accepted Answer · 2016-04-05 22:03:49Z

1

You cannot use operator > to compare int[] types. Java doesn't support operator overloading. Those relational operators (>, <, >=, <=) are applied to numeric primitive data types only.

Do you mean something like grades[r][c] > grades[r][max_test] or grades[r][c] < grades[r][min_test]?

edited Apr 5, 2016 at 22:03

answered Apr 5, 2016 at 21:02

Andrew

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3 Comments

Beeeee Over a year ago

that produces another Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 87 at Grades5.main(Grades5.java:55)

Andrew Over a year ago

@Brent, you're asking about ArrayIndexOutOfBoundsException 2 times for a short time, I suggest you reading about this exception and how to work with arrays.

Andrew Over a year ago

@Brent, I don't have to write full working code for you

Beeeee · Accepted Answer · 2016-04-05 21:31:44Z

0

This works because grades needs 2 parts when comparing

    if (grades[r][c] > max_test) 
      max_test = grades[r][c]; 
    if (grades[r][c] < min_test)
      min_test = grades[r][c];
      sum += grades[r][c];

answered Apr 5, 2016 at 21:31

Beeeee

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